The SR is more highly developed in fast muscle than in slow muscle, with higher levels of RYR, SERCA, lumenal Ca, and a higher DHPR/RYR ratio, all of which promote the development of a larger, faster intracellular Ca transient in fast muscle, which is important for quick, forceful contraction. In addition to the differences between fast and slow fibers just noted, other muscle proteins are also expressed in a fiber type–specific manner. Such proteins include the three troponin subunits, tropomyosin, and C protein. The differential expression of troponin and tropomyosin isoforms influences the dependency of contraction on Ca++ . Slow fibers begin to develop tension at lower [Ca++] than fast fibers do. This difference in sensitivity to Ca++ is related in part to the fact that the troponin C isoform in slow fibers has only a single low-affinity Ca++-binding site, whereas the troponin C of fast fibers has two low-affinity binding sites. Changes in the dependence of contraction on Ca++ , however, are not restricted to differences in the troponin C isoforms. Differences in troponin T and tropomyosin isoforms are also found. Thus regulation of the dependence of contraction on Ca++ is complex and involves contributions from multiple proteins on the thin filament. Phosphorylation of the regulatory light chain of myosin by Ca++/ calmodulin-dependent myosin light chain kinase, however, can increase Ca++ sensitivity of contraction, particularly in fast muscle fibers (partly because of the reported higher activity of myosin light chain kinase in fast muscle fibers).
•Fig. 12.16. Increasing the Frequency of Electrical Stimulation of Skeletal Muscle Results in an Increase in the Force of Contraction. ThisisattributabletoprolongationoftheintracellularCa++transientandistermedtetany. IncompletetetanyresultsfrominitiationofanotherintracellularCa++transientbeforethemusclehascompletelyrelaxed.Thusthereisasummationoftwitchforces.Seetextfordetails.Action potential Time 1 sec Myoplasmic Ca++ Incomplete tetanus Twitch force Tetanus force
Modulation of the Force of Contraction
A simple means of increasing the force of contraction of a muscle is to recruit more muscle fibers. Because all the muscle fibers within a motor unit are activated simultaneously, a muscle recruits more muscle fibers by recruiting more motor units. As already noted, muscle fibers can be classified as fast-twitch or slow-twitch. The type of fiber is determined by its innervation. Because all fibers in a motor unit are innervated by a single α motor neuron, all fibers within a motor unit are of the same type. Slow-twitch motor units tend to be small (100 to 500 muscle fibers) and are innervated by an α motor neuron that is easily excited (see
Table 12.2 ). Fast-twitch motor units, in contrast, tend to be large (containing 1000 to 2000 muscle fibers) and are innervated by α motor neurons that are more difficult to excite. Thus slow-twitch motor units tend to be recruited first. As more and more force is needed, fast-twitch motor units are recruited. The advantage of such a recruitment strategy is that the first muscle fibers recruited are those that have high resistance to fatigue. Moreover, the small size of slow-twitch motor units allows fine motor control at low levels of force. The process of increasing the force of contraction by recruiting additional motor units is termed spatial summation because forces from muscle fibers are being “summed” within a larger area of the muscle. This is in contrast to temporal summation, which is discussed later.
Action potentials in skeletal muscles are quite uniform and lead to the release of a reproducible pulse of Ca++ from the SR (